1. Field of the Invention
The present invention relates generally to an apparatus for sensing toner concentration in a container of developer material, and more particularly to a technique employing an active magnetic force to compress a preselected volume of developer material so that a representative permeability measurement of the developer material in the container can be obtained.
2. Description of the Prior Art
In an electrophotographic printing machine, the photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing marking or toner particles into contact therewith. This forms a powder image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the marking particles thereto in image configuration.
Various types of development systems have hereinbefore been employed. These systems utilize two component developer mixes or single component developer materials. Typical two component developer mixes employed are well known in the art, and generally comprise dyed or colored thermoplastic powders, known in the art as toner particles, which are mixed with coarser carrier granules, such as ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. When the developer mix is brought into contact with the charged photoconductive surface the greater attractive force of the electrostatic latent image recorded thereon causes the toner particles to transfer from the carrier granules and adhere to the electrostatic latent image.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner particles of a color complimentary thereto. This process is repeated a plurality of cycles for differently colored images and their respective complimentarily colored toner particles. For example, a red filtered light image is developed with cyan toner particles, while a green filtered light image is developed with magenta toner particles and a blue filtered light image with yellow toner particles. Each single color toner powder image is transferred to the copy sheet superimposed over the prior toner powder image. This creates a multi-layered toner powder image on the copy sheet. Thereafter, the multi-layered toner powder image is permanently affixed to the copy sheet creating a color copy. An illustrative electrophotographic printing machine for producing color copies is the Model No. 1005 made by the Xerox Corporation.
It is evident that in printing machines of this type, toner particles are depleted from the developer mixture. As the concentration of toner particles decreases, the density of the resultant copy degrades. In order to maintain the copies being reproduced at a specified minimum density, it is necessary to regulate the concentration of toner particles in the developer mixture. Moreover, sensing of toner concentration provides valuable input for process control of the electrophotographic printing machine. Toner concentration can be regulated by various known techniques, one of which includes monitoring an electro-magnetic property of the developer, such as permeability, permittivity or conductivity, to obtain information regarding the carrier-toner ratio. The following references may be of pertinence to the present disclosure:
U.S. Pat. No. 3,572,551 PA0 U.S. Pat. No. 3,698,926 PA0 U.S. Pat. No. 3,707,134 PA0 U.S. Pat. No. 3,802,381 PA0 U.S. Pat. No. 3,970,036 PA0 Tso, T.T.
Patentee: Gillespie PA1 Issued: Mar. 30, 1971 PA1 Patentee: Furuichi PA1 Issued: Oct. 17, 1972 PA1 Patentee: Gawron PA1 Issued: Dec. 26, 1972 PA1 Patentee: O'Neill et al. PA1 Issued: Apr. 9, 1974 PA1 Patentee: Baer et al. PA1 Issued: Jul. 20, 1976 PA1 Toner Concentration Meter PA1 Xerox Disclosure Journal, Vol. 5, No. 3 at p. 315 PA1 Published: May/June 1980
The above-cited references can be summarized as follows:
U.S. Pat. No. 3,572,551 discloses an apparatus for monitoring and controlling the concentration of toner in a developer mix. The method implemented by the apparatus comprises the steps of (1) passing samples of the developer mix past, and adjacent, a coil connected in an AC circuit, whereby the inductance of the coil varies as a function of the concentration of the toner in the samples of the mix and the AC circuit provides output signals which vary with the concentration of the toner in the samples, and (2) comparing the output signals to the reference signals of known concentrations of toner in the mix, whereby the concentration of toner in the samples is determinable.
U.S. Pat. No. 3,698,926 discloses a method and apparatus for supplementing toner in electrophotographic machines. The apparatus comprises a source of toner, a container containing a developing agent for applying the same onto latent images, a movable impedance element which varies its impedance in accordance with the percentage content of the toner in the developing agent and means responsive to the variation in the impedance of the variable impedance element to supply the toner to the container from the source so as to maintain the percentage content of the toner at a constant level.
U.S. Pat. No. 3,707,134 discloses an apparatus for monitoring and controlling the ratio of toner-to-carrier particles of a developer mix comprising an inductive sensing coil having an iron core. The coil is placed in the surroundings of a developer apparatus of an electrostatic copying machine so as to be in contacting relation with the developer mix containing toner and magnetizable carrier particles. The inductive reactance of the coil is a function of the amount of magnetizable particles per toner particles in the mix. Thus, as the toner is depleted, the inductance of the coil changes. The frequency of an oscillator circuit connected to the coil changes as the inductance of the coil is varied. The change in frequency produces a corresponding output of additional circuitry which in turn operates a toner dispenser unit, causing toner to be added to the mix to restore the toner-to-carrier ratio to a predetermined level.
U.S. Pat. No. 3,802,381 discloses an apparatus for measuring the ratio of ferromagnetic carrier particles to toner particles in an electrostatic printing machines. Generally, an electric or magnetic field is established in the area of a quantity of developer mix and a measurement of a parameter, such as magnetic permeability, is employed to indicate the need for a greater or lesser percentage of toner or carrier in the mix. In the preferred embodiment of the disclosed invention, a trough, located in the path of movement of mix within the printing machine, provides a build-up of mix in which one of the fields may be established. In one aspect of the disclosed invention, first and second coils are positioned in the trough, the coils being disposed in air or wrapped about a core. The first and second coils are respectively coupled to an AC generator and an AC meter. In operation, a magnetic field is established in the trough so that magnetic permeability, an indicator of toner concentration, can be measured as the mix flows through the trough.
U.S. Pat. No. 3,970,036 discloses a toner concentration detector in which developer removed from a photoreceptive member after developing is directed through a duct. A coil surrounding the duct comprises an element of a sensing oscillator, the frequency of which is compared with that of a tunable reference oscillator to provide a frequency difference signal, which signal is a measure of the relative proportion of toner to carrier in the developer. This toner concentration signal is employed to actuate a toner replenishing system to feed toner from a supply of toner to a developer.
The Xerox Disclosure Journal discloses a toner concentration meter system comprising a tube located in the air core of a transformer. In operation, the primary windings of the transformer are excited with an alternating current to produce an alternating current output in the secondary windings. The secondary windings are coupled with a tuned secondary circuit having a characteristically sharp resonance point. Since the resonant frequency varies with toner concentration, the concentration of a given developer can be determined by suitable adjustment of the driving frequency of the system.
Except for U.S. Pat. No. 3,707,134, the above references disclose a "passive" magnetic sensors that are capable of determining toner concentration by measuring the magnetic permeability of developer flowing through a tube or the like. It has been found that such passive sensors have a sensitivity to developer flow variations, and accordingly are subject to undesirable levels of "noise" or error. Other problems, such as developer aging, non-geometric packing fractions and changes in the environment can also have an adverse effect on the performance of such passive sensors.
While the sensing arrangement of U.S. Pat. No. 3,707,134 ('134 patent) avoids some of the above-mentioned problems, it is relatively complex in design, and can yield inaccurate results. In particular, the sensor of this arrangement is positioned adjacent a magnetic brush and can thus become contaminated by stray developer material. Moreover, unless the layer of developer on the brush is closely metered, inaccurate toner concentration readings can be obtained. Finally, the circuitry for the arrangement of the '134 patent includes many components, and is thus relatively expensive to manufacture.
It would be desirable to provide a sensing apparatus that is capable of measuring magnetic permeability of developer material without being subjected to undesirable levels of noise. Moreover, it would be desirable to provide a relatively inexpensive sensing apparatus that is both easy to implement and free from a contaminating environment.